I think Chet15 has is right when he writes..."The highest 4" I have noted is 503-58632 and the earliest 4.2" noted is 503-58641"
I have seen a Redhawk slightly below 503-58490 manufactured 12/2011 that had a 3.85" barrel (measured end of muzzle to beginning of forcing cone), and have seen one above 503-64000 that had a 4.2" barrel (actually measured slightly above 4.2").
Yes, there is a slightly noticeable difference in overall look to the revolver. I feel the 4.2" one looks better than the less than 4" barrel Redhawk.